1. Field of the Invention
The present invention relates to a toner density control method for an image recording apparatus, e.g., an electrophotographic copying machine, and an apparatus for practicing this method.
2. Description of the Prior Art
In an image recording apparatus, e.g., an electro-photographic copying machine and a facsimile apparatus, an original is irradiated with light, an electrostatic latent image is formed on a photosensitive body by light reflected by the original, and this image is recorded on recording paper in the form of a visual image, or an electrical signal representing an image supplied from outside is recorded on recording paper in the form of a visual image.
In an image recording apparatus of this type, a visual image is formed by using a two-component developing agent consisting of a toner and a carrier, and the visual image is recorded on the recording paper. A toner density affects the density of a copy image. Hence, in order to always obtain a recording image having a predetermined density, the toner density is detected and control is performed such that when the toner density is decreased, the toner is replenished so that the toner density falls within an appropriate range.
Various methods have been conventionally known to detect the toner density. For example, according to Japanese Patent Publication No. 64-5299, when a developing agent is to be replaced, a reference value serving as a reference of density control is selected by a selector switch, and toner density control is performed on the basis of the selected reference value. According to another method, the density of a toner image formed by a small piece having a reference density is optically detected. According to still another method, a magnetic field is formed by an RF voltage, this magnetic field is caused to partly affect a two-component developing agent, and a change in magnetic permeability per unit volume of the developing agent is detected, thereby detecting the toner density.
Generally, in image recording apparatuses each using a two-component developing agent, when a control system to change a toner supply amount or to stop the operation of each apparatus (in other words, to protect the apparatus) in accordance with a toner density is employed, a difference between the output value from a toner density sensor with respect to the reference toner density and the output value from the toner density sensor with respect to each toner density sometimes varies from one machine type to another. The level of the toner density needing toner replenishment varies due to this variation.
As a result, replenishment is delayed when it is needed, and the toner density of the developing agent is decreased, causing a decrease in image quality and degradation in developing agent.
FIG. 2 shows the relationship among an output voltage Vo of a toner density sensor, a substantial toner density Tc, a detected toner density Td when a control voltage (the voltage, generated by the toner density sensor, for controlling the strength of the magnetic field) Vc of a standard toner density sensor is about 7.0 V as its general value or changed to 6.75 V or 7.25 V). Referring to FIG. 2, the substantial toner density Tc is the actual toner density. Since the apparent voltage Vo obtained by the toner density sensor changes only by 0.01 V, the detected toner density Td cannot substantially detect this change. In order to change the output voltage by 0.1 V, as in the standard toner density sensor, by using this toner density sensor having a low sensitivity, the substantial toner density Tc must change to 5.0%.
For this purpose, assume that the toner density is controlled by using a toner density sensor having a low sensitivity S. Then, the recording apparatus detects that the toner density is still at a normal value of 6.0% when toner replenishment is actually needed, and the toner is not replenished. The toner is actually replenished after the substantial toner density Tc is considerably decreased. For this reason, the number of times of stirring the toner and carrier after the toner is initially replenished and before the toner is replenished next becomes larger than that obtained when the standard toner density sensor is used, and carrier particles collide to be damaged during this stirring. Then, the durability of the developing agent is degraded, and the toner is stirred before new toner replenishment (not charged since the toner is not yet stirred), causing an increase in charge amount of the toner and carrier. When the charge amount is increased, the flowability of the toner is decreased. The toner is then attached to the toner density sensor to cause erroneous detection, or the stirred toner and the newly replenished toner are not mixed well, forming toner particles (a toner mass) to be attached to the drum to contaminate the recording paper, thus disabling normal recording.
Inversely, when the sensitivity S of the toner density sensor is excessively high, despite that the toner density can originally be controlled by about 4 to 8% by using the toner density sensor, the range of the toner density control is narrowed, or the toner density sensor causes a malfunction because it can be easily affected by a power supply ripple or the noise. Although these phenomena can be prevented by increasing the capacitance of a smoothing capacitor or by using a noise filter, the toner density is largely influenced by the environmental conditions. No problem arises in toner replenishment since an appropriate amount of toner is replenished.
The above description applies to a case when the sensitivity of the toner density sensors varies. However, even in the standard toner density sensor, when originals having low densities (e.g., originals containing only ruled lines or a small number of characters) are continuously recorded, the toner consumption becomes smaller than that obtained when originals having ordinary densities are recorded, and toner replenishment to the developing chamber is not performed until the detected toner density is decreased to a predetermined value. For this reason, the number of times of stirring the developing agent after the toner is once replenished to the developing chamber and before it is replenished again becomes larger than that obtained when originals having high densities are recorded. In this case, therefore, the durability of the developing agent is degraded and the charge amounts of the toner and the developing agent are increased, in the same manner as in the case wherein the toner density sensor having the low sensitivity S is used. Then, the flowabilities of the toner and the developing agent are decreased, the toner is accumulated on the toner density sensor, or toner particles are formed to be dropped on the recording paper, thus disabling normal recording.
A toner density sensor incorporates a coil for generating a magnetic field to be applied to the two-component developing agent, and a coil for detecting a change in magnetic permeability of the developing agent by utilizing this magnetic field, and detects a density in toner from the obtained magnetic permeability.
FIG. 1 is a schematic diagram for explaining the principle of the toner density sensor.
A magnetic field generating coil La for generating a magnetic field and a detection coil Lb are arranged in the toner density sensor to oppose each other, and lines Ba of magnetic force as indicated by solid lines are generated by the coil La. If a developing agent (carrier) is present between the coils La and Lb, the paths of the lines Ba of magnetic force are changed to alternate long and short dashed lines to extend through the coil Lb. Thus, the presence and amount of a magnetic body can be detected by the coil Lb.
In testing toner density sensors before shipment, for example, a ferrite core is used as the dummy developing agent while the control voltage Vc of each toner density sensor is set at a constant value. The ferrite core is set close to the detection coil Lb, and the thickness (i.e., corresponding to the density of the developing agent) of the ferrite core serving as the dummy developing agent is changed to obtain an output voltage Vo. Only the toner density sensors, the gradients of the characteristic lines of the output voltage Vo of which fall within a predetermined range, are selected as non-defective products.
Toner density sensors that passed such a test are mounted in the image recording apparatuses on an assembly line.
However, even a toner density sensor that has passed the test and is mounted in the image recording apparatus cannot sometimes perform precise density control when used actually depending on the environmental conditions, e.g., the temperature and humidity.
Even if the toner density sensor is normal, if the user erroneously uses a developing agent available from a different company, density control cannot sometimes be sufficiently performed due to the different characteristics.
A toner density sensor used in an image recording apparatus of this type has a variation in the change amount in output voltage Vo of the toner density sensor with respect to the change amount in substantial toner density Tc, i.e., has a variation in the sensitivity S, and sometimes detects a toner density change different from the actual toner density change. When the variation in the sensitivity of the toner density sensor is small, no problem arises in controlling the toner density by using this toner density sensor. However, when the variation in the sensitivity is large, especially, when the sensitivity is excessively low, normal toner density control becomes difficult to perform.
The problem arising in toner density control by using a toner density sensor having a low sensitivity will be described with reference to FIG. 3.
Referring to FIG. 3, the axis of abscissa represents the substantial toner density Tc, and the axis of ordinate represents the output voltage Vo of the toner density sensor. The sensitivity characteristics of the standard toner density sensor are represented by a solid line (TYP in FIG. 3), and those of toner density sensors (two types) each having a low sensitivity are represented by broken lines (MIN(1) and MIN(2)) in FIG. 3. Assume that a standard toner density sensor having a sensitivity S of, e.g., 0.5 V/% (a change in output voltage Vo with respect to the substantial toner density change of 1% is 0.5 V) is used. When the detected toner density Td is changed from 6.0% to 5.8%, the output voltage Vo is increased from 1.6 V to 1.7 V by 0.1 V, as is seen from FIG. 3, and when the output voltage Vo is changed by 0.1 V, the toner is replenished so that the substantial toner density Tc becomes 6.0%, so that recording is performed at a normal density.
However, when a toner density sensor having a low sensitivity S (e.g., 0.2 V/%) is used, since its characteristic line is MIN(1), even if the detected toner density Td is similarly changed from 6.0% to 5.8%, the output voltage Vo is changed by only 0.02 V. It is difficult to detect this change. In order to change the output voltage by 0.1 V by using this toner density sensor having the low sensitivity, as the standard toner density sensor, the substantial toner density Tc must change to 5.5%. When a toner density sensor having a lower sensitivity S (0.1 V/%) is used, since its characteristic line is MIN(2), even if the detected toner density Td is similarly changed from 6.0% to 5.8%, the output voltage Vo changes only by 0.01 V. It is substantially impossible to detect this change. In order to change the output voltage by 0.1 V, as in the standard toner density sensor, by using this toner density sensor having the lower sensitivity, the substantial toner density Tc must change to 5.0%.
Assume that the toner density is controlled by using a toner density sensor having a low sensitivity S. Then, the recording apparatus detects that the toner density is still at a normal value of 6.0% even when toner replenishment is actually needed, and the toner is not replenished. The toner is actually replenished after the substantial toner density Tc is considerably decreased. For this reason, the number of times of stirring the toner and carrier after the toner is initially replenished and before the toner is replenished next becomes larger than that obtained when the standard toner density sensor is used, and carrier particles collide to be damaged during this stirring. Then, the durability of the developing agent is degraded, and the toner is stirred before new toner replenishment (not charged since the toner is not yet stirred), causing an increase in charge amount of the toner and carrier. When the charge amount is increased, the flowability of the toner is decreased. The toner is then attached to the toner density sensor to cause erroneous detection, or the stirred toner and the newly replenished toner are not mixed well, forming toner particles (a toner mass) to be attached to the drum to contaminate the recording paper, thus disabling normal recording.
Inversely, when the sensitivity S of the toner density sensor is excessively high, despite that the toner density can originally be controlled by about 4 to 8% by using the toner density sensor, the range of the toner density control is narrowed, or the toner density sensor causes a malfunction because it can be easily affected by a power supply ripple or the noise. These phenomena can be prevented by increasing the capacitance of a smoothing capacitor or by using a noise filter. Toner replenishment largely depends on environmental conditions. However, no problem arises in toner replenishment since an appropriate amount of toner is replenished.
The problem posed by different sensitivities of the toner density sensors is similarly posed when the developing agents have different compositions. This is because when the compositions of the developing agents are different from each other, the relationship between the control voltage and output voltage of the toner density sensor does not sometimes become linear.
For example, when the control voltage-output voltage characteristics of a toner density sensor is measured by using a developing agent available from a company A, the characteristic line becomes substantially vertical (the output voltage is increased only by slightly increasing the control voltage). However, when a developing agent available from a company B is used, the characteristic line is almost saturated (the output voltage is not substantially changed even if the control voltage is changed). Then, with the developing agent of the company B, not only normal toner density control cannot be performed, but also machine breakdown may occur.